Learn Portuguese (Portugal): O faroleiro tem vindo a observar as gaivotas. - The lighthouse keeper has been observing the seagulls.

It expresses an action that started in the past and is continuously happening up to the present, just like the English "has been observing." In European Portuguese, this continuous progression is formed using the present tense of ter (tem) + vindo (the past participle of vir) + a + the infinitive of the main verb (observar).

Could I just use the normal present perfect and say tem observado instead?

You could, but there is a subtle difference in feeling. Tem observado usually implies a repeated, iterative action over a recent period (meaning he has observed them multiple distinct times). Using tem vindo a observar strongly emphasizes the continuous, ongoing, and uninterrupted nature of the activity.

Does the word vindo (come) imply that the lighthouse keeper is physically moving while observing?

No. In this structure, the verb vir (to come) loses its literal meaning of physical movement. It acts purely as an auxiliary (helping) verb to show the progression of time, metaphorically bringing the action from the past right up to the present moment.

The current lesson focuses on dever de for probability. How would I change this sentence to mean "The lighthouse keeper MUST have been observing the seagulls"?