Slim, you are probably right. I had "pop up" traffic appear below directly below me when IFR in that region which caused the controller to issue a traffic alert, and as you say TIS wouldn't have seen it either.
The problem with a FIS it is not a reliable source of traffic information and may lead to a false sense of security, so is worthless other than to give altimeter settings IMO. A RIS is valuable but if you're just going for a jolly, say to practice a bit of upper airwork, don't really know where you're going, other than South, and don't know what time you'll be back then it is unlikely a RIS would be given on a gin clear day, or it would be terminated pretty quickly if they are busy.
I have mode C and transpond every time. I have a radio and maintain a listening watch to the relavent ATC unit, but don't always bother to call them unless enroute somewhere....otherwise my call would be along the lines of "GABCD is out of blah, local flight, dunno what altitude, probably going south for a bit, may be about an hour, request RIS"....