Kamasutra 2.3.31

2.3.31

saṃvāhikāyās tu nāyakam ākārayantyā nidrā-vaśād a:kāmāyā iva tasyorvor vadanasya nidhānam ūru-cumbanaṃ cety ābhiyogikāni.

There is also a verse on this subject as follows:

When at night at a theatre, or in an assembly of caste men, a man coming up to a woman kisses a finger of her hand if she be standing, or a toe of her foot if she be sitting, or when a woman in shampooing her lover's body, places her face on his thigh (as if she were sleepy) so as to inflame his passion, and kisses his thigh or great toe, it is called a "demonstrative kiss."