Bluetooth icon keeps randomly disappearing from menu bar in High Sierra
Using inductive logic, and under the assumption that you didn't "un-tick" the box, then it must be software. My best guess would be software associated with a device that uses Bluetooth. One way to check that is:

Hold down shiftoption while clicking the BT icon in your menu bar. You should see all the usual options, plus one called Debug. From here, things get version dependent, but you should at least have a Debug option giving you more options to reset the Mac's BT module, or any connected Apple BT devices. Some earlier versions of mac os, prior to 10.13.6, are reported to also have a "BT log" option. 

And so this might be the first stop, reset devices one at a time in a process-of-elimination mode. 

If you're more conversant with BT, Apple has some tools that might help pinpoint an errant device causing the "un-tick". One of these tools is referenced frequently, and called Bluetooth Explorer.app (download). It's part of Apple's developer's toolkit. Another tool available in the same download is PacketLogger.app. Bluetooth Explorer.app would start on my system (10.13.6), but refused to display the GUI, making it useless. But apparently it does run on some systems, so YMMV. However the PacketLogger.app started without drama. It has features that may help find a particular message or message type - if you know what to look for :) 

Yeah, I agree, fairly remote possibility that a BT device is the cause, and that these apps will pinpoint it, but if all else fails...