First, stop doing that; it drives trademark lawyers crazy. You can’t “trademark” anything — it’s not a verb — but you can obtain trademark rights in your slogan or logo. When people talk about “trademarking” something, they usually use the term to refer to obtaining a federal trademark registration for the mark. It is important to note, however, that registration is not required to obtain enforceable trademark rights in the U.S.
In the U.S., use of a mark in connection with goods or services creates enforceable trademark rights in the geographic region the mark is used in (in most other countries, trademark rights are obtained through registration). These “common law” trademark rights provide more limited rights than are granted via federal or state registration, but even common law rights can block federal registration of a subsequent confusingly similar mark, and due to the wide reach of the Internet it is possible to obtain common law trademark rights far beyond your geographic footprint.
Common law rights are obviously not detected by a search of USPTO records, but they nonetheless may pose significant obstacles to adoption or registration of a new trademark, or expanding the scope of use of an existing mark. A comprehensive trademark search, reviewed by an attorney, is the recommended way to detect potentially conflicting common law marks.