The container is fire-resistant and waterproof for documents. I have tested signal and that's ok through the layer of alumunium on the bag. I am mainly worried about the often mentioned "outgassing" of batteries in airtight containers. I can get a sensative strips comfort 700 -these are often recommended for outdoors rather than waterproof enclosures for battery sensors. But I don't know whether that's just cos they don't need to be in containers. The aim is I don't want to be opening the container at intervals to check the moisture absorbing packs. I don't mind changing a battery from time to time, as I will be alerted by hubitat when it stops working.
I wonder if there is a sensor that has external contacts. I don't know what kind of container you have, but if there was a sensor that decoupled the battery/electronics and the sensor, with a wire between them - you could put the sensor inside the unit, seal up around it, and have the main unit outside of it.
I was thinking along those lines as well. There are leak sensors that do this, but I'd really want to know about humidity before the inside of the container turned into a bath 
The container is like one of those fireproof document wallets with a silicon skin outside. They usually tell you to open them at least once a month to stop mould, but realistically.. just no.
Depending on the seal of your container, can you fit a #30 solid wire past the seal? Wire wrap wire is only 1/2 mm OD (including the insulation). You could keep the battery outside and not have to worry about outgassing (if it really occurs) or changing batteries.
Yeah, need a bit of engineering though to take the thing apart. I mean do you know of a suitable device that lends itself to being adapted this way?
I have Zigbee sensors in my freezers that provide temperature readouts in case the door is left cracked open or the compressor fails. However, because the gaskets surrounding the door are vinyl and not metal, the freezer box is not quite a Faraday cage.
In the case of a fireproof safe, the door gaskets are likely to as thin as possible and probably made of metalic mesh. Thus, you might have a Faraday cage situation which makes using an internal sensor impossible. If you drill holes to allow the passage of sensor wires, you may nullify the fireproof protection of the safe.
In stead of measuring the humidity inside your safe, you might be better off using a humidity control mixture inside the safe. I keep dry baking soda inside mine to absorb moisture and potential odors. Calcium chloride will also absorb moisture. As long as the seal is intact, there should be little exchange of air from inside to outside. If the humidity level of the room in which the safe is situated has a tendency to be high, get a dehumidifier.
An even simpler method is sealing important documents in gallon size Ziplock freezer bags.
If you have older documents printed on acid based paper, oxygen can cause deterioration of the paper. Thus, you might want to include an oxygen scrubber as well if any of the papers are more than 25 years old. Almost all white papers manufactured since then have been acid-free and should last for 100 years without yellowing. Acid-sized papers start falling apart after about 50 years. Newsprint and paper-back books printed on newsprint deteriorate even more quickly. Thus, if you have newspaper clippings you wish to preserve (for example, wedding announcements and death notices), these should be laminated to exclude oxygen.
I would look at CR2032 (or what ever you use) battery eliminator. Something like this device.
Thanks. That may be useful for something but not for this particular sitation. One thing I didn't mention is that there is not a handy electrical supply where I was planning to keep this, otherwise I could have just slipped in an aeotec multi-sensor that I have several of, doing a wonderful job powered by USB
Regarding the comment above yours, about the dessicants, I can of course do that but you still have to check them regularly, thats the bit I was hoping to automate. Also I have already tested the signal which does come through the container walls.
In fact all things considered I have decided to dispense with the fireproof, waterproof bag and just keep them in a normal grabbable folder in the planned cupboard near the front door. It's just our wills and stuff like that, and since we have smoke alarms in every room the chances of them burning before we've managed to grab them and get out is quite small. If we did forget on our way out, these silly little fireproof bags only last about half an hour anyway. I think the papers are in more danger of self-composting with damp than burning lol - that's just based on reviews I have read about fireproof containers. Apparently they generate their own moisture, something to do with the way they work 
I have a friend who is retired as a chief in the fire department. He said that no matter what claims the manufacturers of fireproof safes make, in a major house fire, papers inside the safe are likely to be badly damaged or destroyed. They might survive a fire confined to another part of the house. The best place for a safe is in the lowest level of the home as heat rises.
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