If it does this consistently, that's where I'd start looking, because it's the cheapest and easiest thing to fix. Oh, before that, also check your system voltage (just in case -- the ABS system is voltage sensitive, and a low voltage -- maybe your alternator is falling down on the job -- will give a warning like that, too.)

There are three sensors -- two in each front wheel's hub, and one in the differential (that's supposed to cover both rear wheels simultaneously -- which is why a 3-channel system isn't quite as good as more costly 4-channel system). They (the fronts, that is) don't usually become defective, but only get crudded up with dirt that impairs their function, and a cleaning lets them recover. I'd do the two fronts first -- pull the caliper and rotor, and you'll expose to see a ring with teeth (resembling a gear) around the hub. You'll also see a small cylindrical object (the actual sensor) behind and adjacent to this ring in the front side. Take spray-on brake cleaner and a toothbrush, and liberally spray this ring and that sensor, and then scrub it with the toothbrush. Get all that road and brake dust off. Then reinstall your brake rotor and caliper, and see if it helped.
Oh, and if you were considering a brake job (new rotor and pads, for example), this is a good time, obviously!

I'm sorry but I can't help you with the rear sensor -- never had to work on it, so I have no personal experience to share.

Good luck.