Only you PB knows for sure, and he likely can't be found or won't remember. Not much telling really unless you dig it up. Are you having some sort of trouble that makes you ask? I wouldn't like mine to be that way, but just curious if there is trouble. It is pure speculation on my part, but it looks possible that the capped line could have been plumbed to the suction there at one time, but it's merely a guess.
I would suggest cutting out that union on the suction of the pump at some point, as it is likely to leak and become a suction side problem. I'm not saying it will, but the potential is high for that to be an issue down the line.