The response is yes! For instance, the 5 planets known orbiting 55 Cancri A follow a Titius-Bode law. According to this law, a planet might be missing at position 5 (if the last planet is at position 6)!

The Titus-Bode law likely represents some kind of tidal/gravitational resonance involving various zones of accretion.   I would suspect this to be relatively Universal and "missing planets" are likely the result of processes similar to that which produced the asteroid belt in our Solar system.

...or perhaps the indication of a (small) planet still udetected, right?

Yes, I guess that both an asteroid belt or a small undetected planet could be the explanation of this putative missing planet.

Titius-Body law is actually an approximate logarithmic relationship. Matter seems to organize in orbits so, that tidal resonances by small numbers become preferred. So - Titius-Body is the rule, we must explain its absence or deviations from it. 

Thank you