I have been redeeming heavily using Thank You points (as well as Ultimate Rewards - both use the same travel vendor), in the eReceipt I have occasionally seen "BULK CHECK" as the payment method - otherwise I tend to see various last 4 digits of credit cards - probably those booked with the BULK CHECK are like you said - now I haven't flown any this year yet that were purchased with either program, my first one will be 2/21, but I just checked (because of this thread) and all my future reservations show credit card digits so hopefully I'm correct in assuming it may be limited to those BULK CHECK flights only... who knows why some are done that way while others are not.
"BULK" eTicket (FIA Card Services):
Credit Card eTicket (Thank You Network):
