Mode S codes starting with "T" and "U"

[Protzenalp]

Spent a few days in North America last week. In the Dallas area  I was in a hotel just south of DFW. I had my kit with me and started picking up tons of messages with codes starting with "T" and "U". I thought this might be a glitch with my kit. Rebooted a couple of times, etc. and still had the problem. A couple of days later I moved to the Raleigh-Durham area and did not have this problem at all any more.

Any ideas what the issue may have been?