In "Who Watches the Watchers," Deanna and Riker have subcutaneous communicators implanted in their skin. In the episode "First Contact," Riker loses his combadge. Is there a reason why they didn't inject subcutaneous communicators for all missions?
Riker's visit to Malcor 3 was only intended to be a very short one which is why his hands and feet were merely hidden by clothing as opposed to being altered to match the native species' features along with his face while the long term observation team members were more comprehensively disguised.
Most likely this also accounts for no one taking to time to implant a communicator.