Can an ODB2 reader detect errors even if there is no check engine light present on the dash?

Many can show pending codes. Pending codes are codes where an improper event has occurred, but not enough to warrant turning on the check-engine light.

Beyond that, to see activity that has not been identified as a problem but might be leading that way, you'll want to look for a scanner. You could use a scanner to see the current activity and compare to known measurements and experience.

To clarify an OBDII reader is just a reader. It really does not detect anything. It reads any codes that are stored in the computer/*. The computer stores codes from sensors that are out of specifications. Some codes will turn the light on immediately, some will wait until there are a certain amount of occurrences. If the computer has stored codes that have not yet turned the CEL light on, the reader should read them.