The space that was used for the texture is no longer locked. The texture data is still there and even the handle still may be used, although it will be removed or overwritten when blitz performs the next garbage collection. I don't know exactly when it's gonna do that, every Renderworld() as Sybixsus said, or only from time to time, or when new memory is allocated. Fact is a texture that was freed is no longer accessable _securely_ cause it's RAM will be recycled soonish.
Therefore I'd reccomend if you remove a texture, always set its handle to zero at the same time, this way you will always be able to check if a texture exists or not.
Therefore I'd reccomend if you remove a texture, always set its handle to zero at the same time, this way you will always be able to check if a texture exists or not.