Welcome to the forum.
You may be right that there is a period in which IgM but not IgG antibodies would be present. To my knowledge, there are no data on whether early positive results with the antibody tests -- in the 10-30 day range -- result from detection of IgM, IgG, or both. The important point is that nobody being tested for HIV should get hung up on such details. The main difference between earlier ("2nd generation") and newer ("3rd generation") antibody tests is that 100% reliability is reached at ~8 weeks versus ~6 weeks.
Here is teh link to a recent thread that further discusses seroconversion time. It should answer any further questions you may have about it.
http://www.medhelp.org/posts/HIV-Prevention/what-is-the-window-period-for-hiv/show/1704700
Regards-- HHH, MD