I'm sure a lot of laptop computers containing screens and keyboards have been used as evidence. It may be a little too much to ask FBI agents to only take that which contains data, when it's not necessarily always completely apparent.
Indeed. Newer computers can look like a plain old monitor and have an entire system onboard. This is typically obvious when it's a mac, but would an FBI agent pick out this System76 computer as not being "just a monitor"? Who knows