I had some people replace a log without asking me, and they just chucked it away. I only work round the corner from the cache, and it gets very busy so am quite used to replacing it every six months or so. I was pretty miffed to be honest. If the log was a damp mush, or the cache obviously not looked after then I could understand it but this one wasn't. There was also enough room to put their new (much smaller) log in with the old log.
I sent them a polite message asking why & what they'd done with it and they replied along the lines of "Probably binned it, whatevs." I actually doubt if it was even full, even it if all the spaces were taken there still would've been room to sign it somewhere.
There's no point in anyone signing a log if someone else is going to turn up and just bin it as they seem fit. I'm not saying I check all the names of course, but I think it's pretty poor form to interfere with a cache without asking the CO, unless there is some dire need.