Protein that exit from the cell - as marker
You can always express GFP or your favorite protein in a secretory vector like pSecTag or pSecTag2 from Invitrogen/Life/ThermoFisher/SuperUltraBioMegaMart. There are probably others, but those were the first ones I came across. They contain the mouse Ig&kappa; signal sequence for efficient secretion (so the website says, I've never used it).

I don't know the details of your experiment, but are you sure you want to use a secreted protein as a transfection marker? Remember, it's possible that only a small fraction of your cell population will take up the vector (or mRNA, if that's what you're using, but there are issues with that), but you will see secreted protein regardless of the number of cells that take it up.