My factory installed panic alarm on a 1998 mustang goes off after the
vehicle sits locked for periods of time. Short of leaving it at a
dealer all day to troubleshoot (nobody has dealt with this it seems)
I've done everything else (new battery etc). Its happening more
frequently and am forced to leave it unlocked to prevent. Isn't there
a way to disconnect this
feature? I'd be happy to disable the entire remote if necessary.

Check the fuse diagram in the owners manual for the engine compartment
fuse box and interior fuse box.  Look for a fuse that says security
system or something of the sort. If the security system has its own
fuse, remove the fuse, this will disable the security system.  You
could also run down a wiring diagram for your mustang and disconnect
the power from the alarm, but that is alot of work.