In the US we often use the expression, "Still waters run deep" to describe someone who is unusually quiet and seems to be always lost in thought. The single word philosopher is also used (often in a somewhat sarcastic way) to refer to a contemplative person, or someone who remains silent while others are in a lively or heated discussion:
"Your baby is so quiet - he just sits in his chair."
"Yes, he's a philosopher."
"How could you keep silent while everyone was yelling at that meeting?"
"I'm a philosopher"
A few dictionaries will reference this meaning, although it's mostly colloquial.