Angels reveal prophetic truth from God

 This comes out i Daniel 10 as well as in other places such as Ezekiel, Zechariah and Revelation.

Angels can be encountered without our realising

Hebrews 13:2 Do not forget to show hospitality to strangers, for by so doing some people have shown hospitality to angels without knowing it.
This is clearly a reference back to Abraham and Lot back in Genesis 18 and 19 and is intended to encourage hospitality. However, it does suggest that it is possible to meet an angel and not know, for them to appear incognito. Perhaps the idea is of the stranger brining the angels with him.

Angels rejoice when sinners repent

In Luke 15:10 it says that

... there is rejoicing in the presence of the angels of God over one sinner who repents.

If that is read carefully, it must mean thhat God rejooices when sinners repent. Undoubtedly the angels rejoice too.