Hello! It ought to sound like this: Mary has lost her pen. Have you seen it (somewhere)? I guess, in AE people often tend to use the past simple all the time. Whereas in the BE the distinction is clear. I hope this helps.

Have you seen it anywhere? We use present perfect because although losing the pen happened in the past, the situation is very active because the pen hasn't been found yet.

Mary LOST her pen. HAVE you SEEN it here and there? Which tense SHOULD be used? Present Perfect or PRETERITE TENSE? Thank you.