We can't give out that kind of information, for reasons of confidentiality. Confidentiality is one of the most important principles of mystery shopping, without which the industry could not survive. In order for mystery shopping to be worth anything as a service, and for clients to be able to use mystery shopping to perform a realistic assessment of how their retail workers are performing, mystery shoppers must remain anonymous and maintain confidentiality about their work at all times. This includes what companies shop what clients. A question such as the one you asked is a classic set-up to try to obtain information that should never be disclosed.
The MSPA Code of Professional Standards and Ethics Agreement for Mystery Shoppers explains this and other standards of professional mystery shopping conduct. It can be found at the MSPA website at http://www.mspanorthamerica.com/ethics/ethics1.php.