2004 Sienna fast idle

My wife noted the other day that when starting the van after it has been sitting a few hours, that it idles at just under 2000 rpm. Doesn't do it first thing in the morning though...only after she drives it to work, then lets it sit in the parking lot for about 6 hours. After she drives it a bit, it behaves normally. I haven't been able to catch it messing up myself, but she's been driving the van for a couple years and knows how it normally drives.
I checked for codes but there were none. Checked our Haynes manual which had nothing to offer as far as what to check. Any ideas?
Thanks,
-Mike