I bought a multiple re-entry permit along with my "retirement visa". I left Thailand a few weeks ago and are now about to return to Thailand.

Did I have to mention anything to the immigration officer upon leaving Thailand a few weeks ago? Or not necessary?

No need to say anything. 

OP, just hand pp to io .

Be aware of your expiry date of your reentry permit.  

As you walk away having been handed back the passport pause and check that the io stamped  correct "admitted till date"

nada

Can I add a question to this topic please. If I leave Thailand on my retirement visa with multi entry, how long can I be absent from Thailand? Eg I have had visa since November. If I go to UK now can I stay there 4 months?

1 hour ago, Martin Brit said:

Can I add a question to this topic please. If I leave Thailand on my retirement visa with multi entry, how long can I be absent from Thailand? Eg I have had visa since November. If I go to UK now can I stay there 4 months?

 

  You need to be back in Thailand in time to do your annual renewal of permission to stay.  Aside from that, stay away all you want.

20 hours ago, Martin Brit said:

Can I add a question to this topic please. If I leave Thailand on my retirement visa with multi entry, how long can I be absent from Thailand? Eg I have had visa since November. If I go to UK now can I stay there 4 months?

A re-entry permit should be dated to cover your current permission of stay stamp.
If your current permission of stay (a permit, not a visa) is dated, for example 11th November, then your re-entry permit should also be dated to that same date.

 

On re-entry to Thailand prior to that date, you will be granted permission of stay again until 11th November.