Hmm that is a strange one, do you have TCP state logging enabled on the gateway under Additional Logging Information?
Check the advanced Track properties of the rule in question; even if Track is none, try making sure that both Log Generation boxes are also unchecked as shown here even though they are greyed out:

Also I'm wondering if this could be some kind of bizarre indexing issue, if you bring up ye olde SmartView Tracker do the unindexed logs for the rule with a Track of None show up there as well?
The traffic in question isn't HTTP is it? There is a hidden variable called http_log_every_connection in GUIdbedit that is false by default.
Beyond that probably need to engage with TAC for a kernel debug out on the gateway to figure out why it is logging those.
Attend my online "Be your Own TAC: Part Deux" CheckMates event
March 27th with sessions for both the EMEA and Americas time zones