～ていた Was doing ~

To convey that '(A) was happening', or that somebody 'was doing (A)' in Japanese, ていた (or ていました) will be used. ていた is a construction which uses the conjunction particle て, and the ichidan verb いる (to exist), in its past tense form いた.

Sometimes ていた is used to express things that are still ongoing, but the speaker is not sure about, or would like to politely advise the listener about. This is very similar to the way in which English uses '(A) was doing (B)'.

In this example, although the action of 'sitting' is not finished, we can imagine that the speaker is suggesting that 'dad was sitting here, and he still is, but he just went to get a drink'.

The same kind of sentence could be used when the speaker has walked away from where their dad was sitting, and doesn't actually know if he is still sitting there or not.