Diagnostic codes can be stored even if no MIL is present. Certain specifications of the OBD standard indicate that the MIL will illuminate if the problem can affect vehicle emissions. Some other codes may not directly affect those, and so will not light the MIL.
As far as the problem, check to make sure you didn't knock any vacuum lines off. Make sure all the wires you disconnected are snug. Check your wires, too. Might've opened up or cracked some insulation or the wires themselves going to the plugs, depending on the last time they were replaced you might have that ahead of you.
Vehicle re-learn is usually complete within 2 - 3 driving cycles. Most major parameters are restored within the first 20 minutes of normal driving. But OBD I systems aren't as strict nor as informative as the newer OBD II specifications.