4 Yr. Old Child May Have Swallowed A Lego Part

4 yr. old child may have swallowed a Lego part 4 days ago. Was x-rayed but did not show up. Have monitored bm's buy nothing has shown up. Bm's are regular (2-3 x's a day) and normal.
any suggestions as to next course of action. Could it still be in stomach after 4 days? child said he did swallow it.