You don't think a heroin addict who couldn't come up with the money for their next buy isn't going to go check every dead drop they ever bought at on the off chance that there is something there?
I mean, just thinking of the old espionage meaning of dead drop, if you have dead drops or safe houses or secret meeting locations that you could only use once a year, to avoid patterns, then researching them would become tremendously expensive. A full time job, really, probably for several people (and someone in logistics managing them all). I can't see how that'd be much different for criminals, and how do you make a profit at that? At least in the espionage case, you would have been State sponsored. You don't have to make money.