I had a company build out a stage design and paint it themselves. The client that hired them to build the stage later hired me to come in and do all the custom mural work to make it look correct. I took pictures for my website and denoted exactly what I did and what they did. ... He took pictures of his finished work and later came back and retook pictures of my work and posted my artwork on his stage and acted like it was all his. I saw it on his web and brochures. Never giving credit where credit was due.
I thought long and hard about what to do because I own the rights to all my mural work. The client just purchases a "product", the mural, but holds no rights unless I
sign over "authorship". I would suspect that a custom
sign would be similar. You own the rights to the design. All of that is per my lawyer...
I would approach and ask for an explaination. It would end the speculation and the gut feeling you have right now.