The initial wave thanked loyal customers, now others seem to be benefitting.
Last month, Sony very quietly sent out huge batches of $10 credits through the PSN messaging system. The messages described the credit as a "thank you for being a loyal customer and fan of the PlayStation Network." As with similar promotions, users needed to check their PSN message box, and the credit was set to expires unless redeemed and thereby added to the user's wallet. (Banked, rather than spent.)
Reports of a second wave are now cropping up, and it seems that less frequent PSN users are the beneficiaries. It is likely that users are supposed to be encouraged to utilize PSN with the credit, however, if the credit is not added to the user's funds before the expiration date (03/25/13 in the latest instance), then the credit will no longer be available.
The stealthy campaign is considerably widespread, and the best way to check for the credit is through a PS3, PS Vita, PSP. From the XMB the users should navigate to the right towards the XMB inbox.