No, you're basically spying on them. Which is way worse than a prank. It's an invasion of privacy, and they have a right to know about it.

[He leans back against the table behind him, arms crossing over his chest.]

Why is it so important that you keep it a secret, exactly? Most people would be glad to hear that you're trying to look out for them. I mean, I assume that if you give a damn about these people then they must at least treat you decently. More than that, they're probably your friends, whether or not either of you admit it. So what are you worried about?