Both my research and my experience are that while occasionally non-cohanim adopted a cohan surname for reasons as offered above...this was rare. In Jewish tradition---and I'm secular so not pushing a religious viewpoint---it was a shanda to "pretend" to be cohanim when you were not. And the genetics bear this tradition out. It is common that cohanim descendants in a single family with a cohanim tradition took different surnames, many times a non-cohanim surname. So a Steinberg or a Goldberg might turn out to be cohan, but hardly ever is a Katz or a Caplan etc. not cohane.
aka Debra Katz
Pacific Beach CA USA