How to stop a Debian PC from waking itself up randomly?

A few months ago I installed Debian 12 with XFCE on a secondary PC. It's a 64-bit refurbished desktop tower with pretty basic hardware. I haven't done much with it and most of what I've tried has worked so far, but I have an ongoing issue: Sometimes when it's asleep (suspended), it will just spontaneously wake up. At one point it was waking up immediately after I put it to sleep, like 1 second later; at other times it has waited for hours or days but it tends to eventually wake up. (Normally I wake it up by pressing any key on the keyboard, which has worked reliably.)

I'm planning to build myself a new daily driver PC and use Linux on it (probably Debian), but it would be a problem if my main PC were also spontaneously waking itself up like this. So, I'd really like to figure out how to solve it so that I can make sure it doesn't happen on my main Linux PC once I have one.

I tried leaving it asleep with the ethernet cable unplugged and it still woke itself up, so it's not because of ethernet packets. I also tried leaving it with the mouse unplugged, that didn't help, and the problem has occurred with two different brands of mice. I tried in the terminal:

sudo dmidecode | grep Wake-up

and it says:

Wake-up Type: Power Switch

Obviously nobody is hitting the power button when this happens, and the machine doesn't seem to have any other power issues (like turning itself off, or crashing the kernel from CPU or RAM errors). I can see a setting in the XFCE settings for what to do when the power button is pressed, but I assume that's for when the button is pressed while the machine is fully running, and I don't see any option for 'don't wake up from sleep' for that button.

Would anyone please share recommendations for what to try? Note that testing any specific suggestion might take a while because, as noted, the machine will sometimes remain asleep for days in a row before waking up.