The sound does occur across word boundaries - "tengo una casa" etc - and in some compound words e.g. estadounidenses, genitourinario, finoúgrio.
Are there any other Spanish words that contain the combination of vowels /ou/ (as a diphthong or in hiatus)?
Note: I'm not including un-assimilated French/English loanwords like coulis, rouge, touche, roulotte, bourbon, house etc as they tend to be pronounced similarly to the original voice i.e. /u/ or /au/ e.g. in sioux and its assimilated form siux, or in output.
Bowen and Stockwell do not list /ow/ (written /ou/ in this note) among their syllabic nuclei. It occurs in admittedly rare forms such as bou 'fishing with a net dragged by two boats' and the proper name Bousoño.
• "A Note on Spanish Semivowels", Sol Saporta (1956)