My wife has been kind enough to review this photo too. She looked at Streak-headed and Coco Woodcreepers in her field guide as those seem to be the only plausible birds looking like your photo at that location, and she says that Streak-headed sounds good. She has seen both species in the wild in Costa Rica.
You may be right. But, there are only three Pine Warbler records for all of Costa Rica in eBird, none with photographs. The closest sightings to Costa Rica are in the Yucatán Peninsula, the Cayman Islands, and Haiti.
I'm thinking (however quickly) that it's a Tennessee Warbler. The undertail pattern is consistent with that species, and the third photo does look like one.
I did that for my second guess today. On the other hand, that move revealed the fourth letter...
(if the above is considered a spoiler, please delete this reply.)