Hesitating on the river bet--Online tell?

[grjr]

In the past two days I've picked up a couple of pots that I was intending to fold on the river against a single opponent who had been betting all the way. Both times the bettor was firing right out but when the river came they hesitated for 4 or 5 seconds.

The first time I actually had the fold box checked but when they hesitated I unchecked it and called. Is this an actual online tell that anybody has used or did I just get lucky here?