I see you figured it out.
OK, a formula approach could be like this:

In E3 (and dragged over and down) this:
=IFERROR(INDEX('Base Plant-Week 20 '!F:F,MATCH($B3,'Base Plant-Week 20 '!$C:$C,0))&"/"&INDEX('SERP Project - Week 20 '!G:G,MATCH($B3,'SERP Project - Week 20 '!$D:$D,0)),"")
This will return, if a match is found, a value from Base Plant and a / and a value from SERP Project. So a value left of the / means it was found on Base Plant and a value right of the / means it was found on SERP Project.