Does Norton only flag your PC as needing a scan when it thinks there is something wrong with it? For example, I just had Norton say "your PC needs to be scanned for spyware and viruses" but it found nothing on a quick scan.
Looking at my history, it seems it may have been an "early-launch anti-malware scan"?
Edit: ignore that last part, that wasn't relevant - it only scanned 50 items so it wasn't that.