What they are doing is the Sunnah and it is correct. the Prophet salla Allahu alaihi wa sallam had said: between every 2 Athans (the Athan and Iqamah) there is a prayer (2 rak’ah nafl).

In all the masjids in my area the brothers and sisters do 2 rakat nafl straight after maghrib salah. Is this correct? because I learnt that one should wait 10 -15 minutes before praying sunnah pray.