And are you positive that there isn't activity within your network? Like someone else on your wifi for instance.
If you're sure that there isn't any activity then it's pretty much a wasted service call. The problem is probably further down the line. If you're able to get over your provisioned speed some of the time then chances are things are fine within your home and with anything that tech would be checking. If they weren't you'd have degraded performance all the time.
When everyone in your area gets home your speed drops. Right? Sounds to me like Comcast needs to deliver more bandwidth to your area or deal with bandwidth abuse in your area. But maybe you're the first to complain so they need to get out there to see for themselves... and then hopefully address the issue. Don't expect resolution today.