Qué calladito te lo tenías
 

English?
Context
When you ask sb sth and nobody says anything about it. Then you learn about it and you say gosh que calladito,lo tenías.
.

Wow, you've been very closed-mouthed about this!

Perfect Polly, thanks.

tight lipped is a similar phrase

Yes, I think far more likely than the other possibility.

Pero en español esta frase tiene un sentido irónico siempre. El uso del diminutivo marca esa ironía.

The phrase lends itself to irony in English as well, but not necessarily.

Saying "calladito" instead of "callado" put some emphasis about a certain degree of stealthiness that exceeds prudence or cautiousness to become a bit malevolent or opportunistic -or anything with a negative connotation-.

I think 'tight lipped' would also have this force. Somebody in a situation where you know they know something, but they are not saying it, for various reasons, often negative.