It sounds like you've already searched the area, which is the first step. I think it's best to spiral outward from where it was parked, since they don't want to tow it any farther than they have to.

When my car was towed, the person I talked to at PPA said that seven blocks in any direction is the rule, but it was three or four blocks away (depending on how you count).

This seems like another opportunity to say that moving somebody's car, for any reason, without making a public record of where you put it, should be punishable by a fine and the revocation of your tow-truck license.