Hi again, and thanks for your very helpful reply.
There can be various causes for this howling and I will go over them with you.
First, she could have diminished hearing, due to her senior age, and cats who are losing their hearing will howl/meow frequently, because their own voice sounds unfamiliar to them and they want to reassure themselves that it is they who are howling.
Older cats are more prone to develop hyperthyroidism and a symptom of this is hypervocalization (more howling, especially at night, but it could be at any time). Here is more information on hyperthyroidism in cats:
Even though she may not have all the symptoms, it would be a good idea to have her evaluated in person by her vet and have senior blood work done, including a thyroid panel.
Some senior cats develop a condition called CDS (Cognitive Dysfunction Syndrome); this is similar to dementia or Alzheimer's in elderly humans. Mookie might find herself in an area of the house that she doesn't recognize/doesn't seem familiar, and panic, calling for you to come rescue her. here is more information about this condition:
Something may have been 'brewing' in that first week, before you went on vacation, and the stress of you not being there, could have exacerbated the problem. She just may feel stressed or anxious due to something you're unaware of. You can get some Feliway plug in diffusers. These emit cat calming pheromones, undetectable by humans, and are very helpful in stressful situations. You can find them in any major pet supply store or order online.
Cats are also masters of disguise, in hiding illness or injury, so feel all over her body for any lumps, bumps, bruises, scratches, and look for any rashes or irritated skin. If she shows any pain response when you press on any of these areas, it bears further investigation.
It would be advisable to have Mookie seen by her vet and have the lab work done, so you can get a diagnosis and then, treatment, if necessary.
For the moment, try the Feliway, and give her lots of attention and love to reassure her everything is okay.
I hope all will be well with Mookie and please let me know how she's doing and what the vet tells you. Thanks!
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