Do you hear anything on the frequency, such as a loud annoying motorboat sound? If so, post a recording. They could have switched to DMR, in which case you would need a few $20 SDR dongles and DSD+, or one of the new hardware scanners which support DMR, or perhaps a DMR radio.
Also, you could run a band-search on the entire business UHF band, perhaps they're using a new conventional frequency.