As a general rule, just like computers, most cameras have some sort of small power source to hold settings and the clock with no main battery installed. A lot of older cameras had a lithium coin battery, and on some it couldn't be changed without serious disassembly. I have a couple of Nikon D1Hs that act wonky(power up but won't release the shutter) and I suspect it may be a dead clock battery but don't know. A little later they started getting put in "hidden" but accessible locations. I suspect most now use some sort of supercapacitor as I don't even remember seeing anything in my newer Nikon manuals about changing the battery.