In talking to one of our HP reps, they also said that some of the HP LaserJet P3015's, will reset themselves after waking up from sleep mode. Unfortunately that always brings the toner level back to full, rather than accurately reflecting the toner level. Not quite your issue, but maybe helpful info.
I've seen that also. I noticed that it seems to be the oldest printers we have. Maybe lansweeper is having trouble interperting the snmp data that printer provides.