SB 8.15.32

evaṁ sumantritārthās te
hitvā tri-viṣṭapaṁ jagmur
 gīrvāṇāḥ kāma-rūpiṇaḥ
evam — thus; su-mantrita — being well advised; arthāḥ — about duties; te — they (the demigods); guruṇā — by their spiritual master; artha-anudarśinā — whose instructions were quite befitting; hitvā — giving up; tri-viṣṭapam — the heavenly kingdom; jagmuḥ — went; gīrvāṇāḥ — the demigods; kāma-rūpiṇaḥ — who could assume any form they liked.
Śukadeva Gosvāmī continued: The demigods, being thus advised by Bṛhaspati for their benefit, immediately accepted his words. Assuming forms according to their desire, they left the heavenly kingdom and scattered, without being observed by the demons.

The word kāma-rūpiṇaḥ indicates that the demigods, the inhabitants of the heavenly planets, can assume any form they desire. Thus it was not at all difficult for them to remain incognito before the eyes of the demons.