1. If you want plenty of options, Ipsos has them. Between the legacy Maritz site, Presto and Kern scheduling (all having Ipsos postings) there are six brands, in my area, with varying degrees of mystery. They have straight mystery shops, mystery/reveal audit and straight audit. Some reach very worth-it pay rates when bonused.
2. Alta360. 3 major brands in my area, also with varying degrees of mystery. A few less photos and much less pay with no reimbursement on most. Bonuses go up a dollar at a time, if at all, and only at the end of a quarter. But on-site time is minimal.
3. Marketforce. Mystery only and mystery/reveal.