"Si det!" can either be translated literally to the English imperative "Say it!", a little less literally to the imperative "Tell me!", or as an idiom/fixed expression to "Who knows!" or "I wonder!".
"I wonder..." in a literal sense can be translated as "Jeg undres..", "Jeg undrer meg (over).." or "Jeg lurer (på)...". However, both of the latter translations tend to take an object, so "Jeg lurer!" doesn't work very well as an exclamation on its own.
The different meanings of this phrase depends on how you pronounce it: Who knows = Si det, with stress on the de in det, and the long e sound, (like you get a bit introvert or meditative as you say it), as an imperative on the other hand you pronounce it differently: Si det! The stress in on both words, and if you should pick to put a nuance more stress on any of them, you should pick the "si". Si det, in impeative means: Tell it! (or Say it!).