The wording of the Sussex Police press releases, their Facebook appeal and responses and the fact that the Surrey and Sussex Major Crime Team are managing the UK end of this investigation, are a key indication that LE enforcement have credible reasons to suspect that there is a criminal/foul play dimension to Danny and Liam's disappearance.
Between Spanish and UK LE, there will be intelligence obtained from various sources, in an effort to build a picture of Danny and Liam's movements, intentions and contacts. This may be information known to LE only, for operational reasons.
If Danny and/or Liam did purchase SIM cards for a Spanish mobile network, then they may also have secondary or 'burner' phones, purchased in UK or Spain. There could be both legitimate and nefarious reasons for this. Hopefully these phones and SIM's have been identified, if this is the case.
The UK LE press releases have asked for sightings of Danny or Liam to be reported via the UK 999 system and to quote Operation Pheasant. This is unusual and I feel another indicator of the suspected criminal/foul play dimension.
Did Danny and/or Liam have dual nationality? It is possible that one or both also had false passport documentation in addition to the passports left at their hotel (presumably GB and NI)? Did either or both have an intention to return to the UK under the radar and avoid having to present entry documents?
From reading these posts, it is apparent that property purchase in Spain, if indeed that was part of their intention, requires identification. Without being familiar with the Spanish property purchase processes, both legal and illegal, I assume that with false documentation it would be possible to purchase in an alias name, for whatever reason?
I do hope that Spanish and UK LE are in possession of additional intelligence that will lead them to Danny and Liam and put an end to the family's torment.
@Laurynpoole if you read this, I think you are a very brave and strong lady.
I can't begin to imagine the effect this is having on you and the rest of the family but you have my utmost respect and heartfelt good wishes with all you are doing for Danny and Liam. Stay strong!
Emotional appeal for information on Daniel and Liam Poole
Missing father and son: More information about their disappearance released