You drive into a short 150-metre tunnel and see no sign showing the tunnel length (L13). Is that correct?

No, sign L13 belongs at every tunnel, regardless of length

Yes, sign L13 only belongs at tunnels longer than 250 metres

No, that is an error by the road authority that you should report

Explanation

False — trap: threshold value dropped. Sign L13 is placed specifically at tunnels longer than 250 metres (Book p.98).