This happens with me as well on a Motorola mixed smartzone system, but it seems like it may be handshake or other trunk-related data being sent, since I always hear the entire conversation between seeing "LNK" on the display.
When I'm monitoring a full P25 system, however, I often see "LNK" for extended periods on some TGs, so they could be data TGs. It seems like the "LNK" feature is similar to Data Skip or detecting when a transmission is encrypted to spare you of ear-shattering noise. It's fairly unpleasant when the occasional snippet of encrypted P25 transmission does get through, especially when wearing headphones.