C8 Pro 2.5.1.140. The small plug icon reads “Show Connect Devices to Your Hub section” Selecting it “Show Connect Devices section now? Hub reboot may be required to rediscover devices.”
What is this?
A forum search yields no results.
I did figure this out but it is interesting that there is no reference to “Connect Devices to Your Hub” Hubitat browser feature by searching Documentation or Forum posts.
There will be. It’s new in 2.5.1, which was just released yesterday and to which you must have just upgraded. 
I’m surprised that it didn’t appear in the beta releases unless I overlooked it.