Normal engine heat siphoning up through the bike parts. Not really as bad as one some bikes. My ST1100 was a lot worse in that kind of heat and it also has the gas tank mostly under the seat just like our CTX bikes. The ST1300 has it worse than the ST1100, but often only noticed on the legs of those who ride without riding over-pants on the hot days.
I can verify that the ST 1300 did have a lot of heat and I agree with Bob that it was because the engine was enclosed in plastic except the tops of the cams. I had reflective aluminum insulation under my body panels to try and reduce the heat to the rider.
When I first got the bike and rode around with the shorty screen, I did not notice any heat and I was looking for it because I was coming from a Tiger 800 which is well known for its engine heat (although not a big deal to me). After installing a Madstad windscreen I started noticing a stream of hot air flowing up from the front of the seat along the front of my body and into my helmet via the chin area. It was very uncomfortable and that's saying a lot because I love hot weather. I only notice this hot airflow on the hotter days and with the windscreen adjusted for a quiet cockpit. Adjusting the windscreen for more airflow to my upper body solves this issue.