If the managing partner is the one who invited the OP, it is probably not an issue that he knows the OP is the shopper. The managing partner is likely the one who has ordered the mystery shops (or one of the management team which has ordered it). The staff at the restaurant, including the restaurant general manager, are the ones to whom the OP should not want to be outed. I would go and enjoy it, but I would do nothing to make myself memorable.....anything that would tip off the staff. Simply getting a free meal after getting bad service will not identify the OP as a mystery shopper. I have had bad service at two restaurants within the last 12 months which were NOT shops In both cases, I posted a negative social media review and was contacted by higher management and given a free meal which I accepted. It is not that unusual for a restaurant's owners/partners to offer free meals to those who have complained about service. The staff is likely used to it.